For bulk devices, punch through stopper (PTS) doping is often required below an active region to prevent leakage/parasitic channel formation. Ideally, the dopant should be isolated below the active channel layer throughout the flow. A known approach makes use of PTS implants, where the implant is done through the active channel layer. Adverting to FIG. 1A (a two-dimensional (2D) cross-sectional view), an active channel layer 101 is formed on a substrate 103, for example, formed of silicon germanium (SiGe). A PTS implant is then performed through the active channel layer 101, as depicted by the arrows 105 in FIG. 1B. Adverting to FIG. 1C, the active channel layer 101 and the substrate 103 are then annealed to remove any defects and to activate the dopants. The range is set to place the implanted dose 107 below the channel 101 and the fins 101′ thereafter, as depicted in FIG. 1D. However, in reality, the implant tail extends into the channel 101, as depicted in FIG. 1E. The dose may further diffuse into the fins 101′ with the integration thermal budget, as depicted in FIG. 1F. As a result of the late PTS implant, the doping tail in the fins 101′ reduces mobility. In addition, a late PTS implant can relax (or amorphize) the fins 101′, which also reduces mobility.
Another known approach includes an early PTS formation via ion implantation (I/I), a doped glass solid phase doping source, or an epitaxially doped process—the goal of each of these is to create a super steep retrograde well (SSRW) where the dopant is intended to remain below the channel layer. FIGS. 2A through 2D schematically illustrate a background process flow for an early PTS implant via I/I, though it is clear that other techniques can be used to introduce dopant prior to channel formation. Adverting to FIG. 2A (a 2D cross-sectional view), a hard-mask 201, for example, formed of silicon nitride (SiN), is formed over a portion of the substrate 203. The substrate 203 is then etched, e.g., to a depth of 40 nm, for subsequent formation of a SiGe channel layer. A low-energy PTS implant is then performed below the surface of the recessed portion of the substrate 203, as depicted by the arrows 205 and the implanted dose or PTS layer 207 of FIG. 2B. The PTS layer 207 is then annealed to activate and recrystallize the layer. Adverting to FIG. 2C, a SiGe channel layer 209 is formed by epitaxial growth on the recessed portion of the substrate 203. Thereafter, the thermal budget, after channel epitaxial growth and shallow trench isolation (STI), diffuses the PTS layer 207 into the channel 209 similar to FIG. 1E, which reduces channel mobility.
A need therefore exists for methodology enabling channel doping control and the resulting bulk device.